Rainwater Harvest: Calculations for Catchment & Cisterns
TL;DR: Calculate rainwater harvesting system capacity using formulas for catchment area, rainfall, and runoff to ensure adequate water supply.
- Calculate roof catchment area in square feet or meters.
- Apply runoff coefficient for net yield estimation.
- Size cisterns for storm events and annual needs.
- Account for first-flush diversion.
- Combine sections for complex roof designs.
Why it matters: Accurate rainwater harvesting calculations prevent undersized systems and ensure reliable water availability for various needs, enhancing water resilience.
Do this next: Measure your roof dimensions to calculate your potential rainwater catchment area.
Recommended for: Anyone planning, designing, or optimizing a rainwater harvesting system, from DIY enthusiasts to permaculture professionals.
This PDF appendix delivers in-depth equations and examples for rainwater harvesting calculations, focusing on catchment area, runoff, and cistern sizing. Catchment area for rectangular roofs: length (m) × width (m) or (ft) × (ft). Key formula: Catchment area (ft²) × rainfall (ft) × 7.48 gal/ft³ × runoff coefficient = gallons. Uses conservative 0.80 runoff coefficient for roofs to estimate net yield, accounting for evaporation and initial losses. Examples: 47 ft × 27 ft = 1,269 ft² footprint; for 9m × long roof, annual net runoff with 0.80 coefficient. Cistern sizing for large storms: catchment (ft²) × storm rainfall (ft) × 7.48 × coefficient. Recommends first-flush of 1-2 gallons per 100 ft². Rules of thumb for impervious surfaces adjust by coefficient (e.g., 0.80-0.95 for roofs). Practical for engineers: compute per rectangle and sum for complex roofs; drip line defines effective area, ignoring slope. Supports resilience by sizing for peak events, ensuring storage captures 80%+ of runoff. Detailed examples like 1,200 ft² roof for storm events provide concrete benchmarks. Ideal for permaculture or off-grid designs, with formulas enabling custom scenarios like drought-prone areas needing oversized cisterns.